1. Field of the Invention
The present invention relates to a method for automatically positioning a workpiece provided with at least one marking, particularly a wafer for integrated circuits which is to be processed in a charged-particle beam apparatus, relative to a scanning field or a mask, in which a scanning beam scans the workpiece to be aligned along a line, i.e., a line scan, and controls a device when the marking is reached, which thereby generates a control signal which is proportional to the deviation of the marking position from a reference position and which drives a device for correcting the position of the workpiece relative to either the scanning field or the mask.
2. Description of the Prior Art
In the charged-particle lithographic fabrication of structures on semiconductor workpieces (wafers), it is necessary to align the workpiece relative to either the structure-generating charged-particle beam or a mask. This is particularly necessary if in multi-layer processes, the individual details of the structure must be superimposed on each other in precise registration. With advancing miniaturization of structures, the requirements for positioning accuracy have increased. For a largely mechanized manufacturing process, automatic positioning methods are therefore of special interest.
An automatic positioning method is already known in the art in which a workpiece provided with markings is scanned by a focussed electron beam along individual lines. See U.S. Pat. No. 3,644,700. A signal generated by the interaction of the electron beam with the surface of the workpiece, for example, secondary electrons, is registered and transformed in signal modification electronics, for example, a threshold discriminator, into a positioning signal which is suitable for further processing. Through the unequivocal correlation between the positioning signal and the space coordinate of the marking on the workpiece by means of the deflection parameters of the electron beam (for example, the deflection voltage or deflection current), the position of the marking on the line scanned is determined by means of a computer. Through comparison with a reference position, the computer generates a control signal proportional to the deviation from this reference position. This control signal then functions to drive appropriate correction elements, for example, an additional deflection system, for correcting the workpiece position.
The disadvantage of the foregoing method is that it requires a considerable amount of electronic circuitry, since the controlled variable must be determined by a computer. In addition, the control signal is not available immediately after a sweep of the scanning electron beam but only after the computer determination is coupled.
In another known automatic positioning method, a square marking is scanned by a fine electron beam probe along a sinusoidal path. See U.S. Pat. No. 3,519,788. The path followed during scanning in this method is one in which the probe leaves the marking during the positive as well as the negative half-wave. Because of the different secondary electron yields on the marking and the remainder of the workpiece, due to different materials, a distinct jump occurs in the magnitude of the positioning signal between locations on the marking and its environment. During the sweep of the beam through a sine wave, two signal jumps are thus generated which correspond to the areas of the path in which the electron beam leaves the square marking. The widths of the signal jumps during the positive and negative half-waves are a measure of the position of the marking relative to the scanning field. Only if the marking is aligned will both jumps be equal. In order to automate this method, the signals generated during the two half-waves are transmitted to a differential amplifier in which rectified values of the signals are subtracted. The d-c voltage so obtained functions as the control signal by means of which the marking can be centered.
While the electronic circuitry for this method is simpler than for the method described previously herein, it is very sensitive to interference. Since the control signal is obtained in practice by integration of the positioning signal, fluctuations in this signal, such as those caused, for example, by dirt on the workpiece or the marking or by fluctuations in the beam current of the scanning electron beam probe, have a pronounced effect. Any deviation of the positioning signal from the ideal signal generates a contribution to the control signal and therefore causes a positioning error.
Due to the sinusoidal scanning of the marking, there is in addition no linear relation in this method between the control signal and the workpiece position deviation. Several sweeps of the workpiece are accordingly necessary to orient the workpiece marking perpendicular to the direction of propagation of the scanning electron beam probe to the center of the raster area, the reference position.